2025 Pompeii Oven Build Log

[david s]

I think your solution is pretty sound, but it may lead to exposed cal sil at the sides of the entry and the stuff is not particularly strong so would be susceptible to knocks and abrasions there. Casting the vent transition is way easier than building that kind of form in brick. It also allows for a big reduction in weight and therefore heat sink.
i’ve found the 3:1:1:1 recipe leads to excessive shrinkage (you don’t want shrinkage cracks), from the high clay portion so I halve it, with much better results. 3:1:1:0.5, but you will need to add the very fine polypropylene burn out fibres for steam spalling protection. A casting thickness of one inch should be sufficient.

[yoboseyo]

Vent Design Questions:

Hey guys, time to design the vent and I am in need of assistance.

The vent construction is going to sit on top of the inner and outer arches. My idea is to stuff leftover pieces of CaSi in the thermal break (cut into L shapes so it fits not only the thermal break but sit between the vent construction and inner arch) to define the flue opening.

The arches were designed this way because the outer arch is normal brick (kilned) not firebrick. Insulating the vent as much as possible from the inner arch minimises heat leakage to the outer arch.

Illustration:


I've done extensive reading of this forum on vent design and this is my design so far:

Vent dimensions:

• Chimney size: 150mm/6"
• Size at opening: 400mm*200mm (more than double the chimney size)
• Reduction to chimney at a 60deg angle (I read that it should be 30deg - not sure whether that's 30 from horizontal or vertical - I assumed taller the better)

Sample Sketchup:


Questions:

1. Any advice/critique on the proposed vent design?

2. What should I construct the vent from? Options are:

• Brick - I may have trouble getting the ideal sloping shape using brick
• Castable
• Steel

3. Can I use this recipe for castable? These are the ingredients I have (castable refractory is not available here) Coarse sand:crushed firebrick:ciment fondu:fireclay 3:1:1:1

4. Should I insulate outside the vent?

5. If not insulated, can I apply the same rendering on the dome, which is mosaic tiles adhered using thinset to the flue? Or will it be too hot? Is there such thing as refractory tile adhesive?

[yoboseyo]

Update:

Completed the brickwork for the outer arch.

Also fixed up the inconsistent thickness of perlite.

I left a 20mm gap between the inner and outer arches for the door. There's going to be a landing slab placed on the bottom row of brick.

There's also a 25mm thermal break between the inner and outer arches, which I will fill with heat-resistant rope.



Put a wedge of brick to straighten the path of exhaust and support the vent

[yoboseyo]

Putting my lovely collection of Canberra reds to good use

[yoboseyo]

Question:
I know compressing blanket insulation reduces its R value, but say if a 2" blanket was compressed to 1", would that insulate better or worse than a 1" blanket?

I'm a bit concerned that I compressed the insulation too much by overhandling it. I only had 2 rolls of insulation (7.2m*1") and while I got 3 layers , I used a lot of offcuts to fill the gaps, and I wound the chicken wire quite tight.



Then for the perlcrete layer, I'm intending to use 1". I didn't use anything to measure the thickness. I thought that 1" is not very thick so I should be able to make it a uniform thickness just by laying it from the base up. Not the case.

Ended up making a cutout to hopefully correct the roundness in the next layer.


It's makeshift, but it seems to work.


Left a hole to fit a vent



This is where it's at now. Going to focus on the decorative bricks for the base, as the outer arch will be laid atop of that. The flue will sit between the inner and outer arches )(I left some space where it will go).

I have not decided how I'm going to build the flue. I will share my ideas and will have some questions.

[yoboseyo]

I screwed up the dome a few courses up by not using the IT because I thought I didn't need it. A compounding of a small error meant that the dome was going to be lower.

I could either correct it, leaving an unsightly shape in the interior, or roll with it. I decided to roll with it, but with the slightly lower dome, I would need a lower opening. I compensated for this by spreading a layer of sand to raise the floor bricks.

In this time, at least I took the opportunity to correct the orientation of the floor bricks into a herringbone pattern. Crisis averted for now, but the masonry work could've been cleaner





Didn't have anything round for the top layer so I made this out of cardboard


Completed dome with raised floor brick (metal strip to temporarily keep the sand in)

[yoboseyo]

Started the masonry work.

I used 3:1:1 sand:fireclay:fondue. I had tested a mix without the fireclay and it was slightly harder to work but the result seemed the same. Both mixes are a lot harder to work than regular mortar.

The inner arch has been the most difficult thing so far, and I wasn't able to get the arch bricks completely evenly spaced and the bottom of the bricks are not perfectly lined up against each other

[yoboseyo]

Finished cutting the floor brick.

Should be able to get the dome going this weekend.

Called up the supplier for instructions regarding my Ciment Fondu - they said to mix it with only 3:1 sand and nothing else.

Anyone have a good recipe for castable with Ciment Fondu?

[yoboseyo]

I drilled the weep holes and raised the CaSi board with some tiles. I was a bit concerned with the lack of structural integrity of CaSi. I ended up putting the tiles about twice as dense as pictured.

Cutting the floor brick, I forgot to lay it at an angle. I figure it'll be fairly flat on the CaSi, and if not I can run a belt sander over the hard edges.

About the mortar - what's the recipe? 3:1:1 sand:fireclay:fondue? These I can get - already have the fondue

[NewEnglandNewb]

yonosey, if you would like to see an example of weep holes through the hearth and tile to raise the insulation to give the water a path to escape, page two of my build thread shows how I did it. Link -> https://community.fornobravo.com/for...e-island/page2

[david s]

You probably won’t know where the steel reinforcing is, so use a small pilot hole, say 1/4”. Which will allow a new hole location if you strike steel as well as reduce the blowing out. It can actually be an advantage because the divot left will create better drainage to each hole. Once pilot holes are drilled you can increase their size to 1/2”.

[UtahBeehiver]

Plus one on JR's comments. 1/2" will still blow out so drill from the bottom of the hearth upward. CaSi can readily absorb water and as mention before, wet insulation is one of the major reason for poor oven performance.

[JRPizza]

I can't tell if you are planning a roof, but if water is allowed to hit your stand, it is likely some will eventually get under the floor and potentially soak your insulation.. Raising the insulation up with tiles and/or drilling weep holes will help mitigate the effects of moisture ingression if it does happen. I did not do either, and although I put up a roof I still managed to soak my insulation boards during the build. It took me several fires to drive the water out and it had to come out through micro-gaps where the render met the hearth. Weep holes would have made the drying much easier.

[yoboseyo]

Need to consider weep holes in the hearth before the start the floor part of the project. I would drill from the bottom since the concrete will most likely blow out, then silicon some window mesh over the holes to keep the bugs from building nest. Weep holes allow moisture to egress out from the hearth area, wet insulation is one of the main reason for poor oven performance. Many builders are also place scrap or cheap ceramic tiles on the hearth in conjunction with the weep holes, this raises the CaSi board of the hearth leaving channels for water to migrate to the weep holes. Now is the time to do these suggestions.

Thanks, I hadn't considered that. Just to understand properly, how exactly would water get in once the whole thing is rendered over?

How big should the weep holes be? If I make them 1/2", will that blow out?

[UtahBeehiver]

Need to consider weep holes in the hearth before the start the floor part of the project. I would drill from the bottom since the concrete will most likely blow out, then silicon some window mesh over the holes to keep the bugs from building nest. Weep holes allow moisture to egress out from the hearth area, wet insulation is one of the main reason for poor oven performance. Many builders are also place scrap or cheap ceramic tiles on the hearth in conjunction with the weep holes, this raises the CaSi board of the hearth leaving channels for water to migrate to the weep holes. Now is the time to do these suggestions.